Accepted Manuscript Title: MoSe2 nanosheet/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) composite film as a Pt-free counter electrode for dye-sensitized solar cells Author: Yi-June Huang Miao-Syuan Fan Chun-Ting Li Chuan-Pei Lee Tai-Ying Chen R. Vittal Kuo-Chuan Ho PII: DOI: Reference:
S0013-4686(16)31403-7 http://dx.doi.org/doi:10.1016/j.electacta.2016.06.086 EA 27527
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
31-1-2016 16-6-2016 17-6-2016
Please cite this article as: Yi-June Huang, Miao-Syuan Fan, Chun-Ting Li, ChuanPei Lee, Tai-Ying Chen, R.Vittal, Kuo-Chuan Ho, MoSe2 nanosheet/poly(3,4ethylenedioxythiophene): poly(styrenesulfonate) composite film as a Ptfree counter electrode for dye-sensitized solar cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.06.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
MoSe2 nanosheet/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) composite film as a Pt-free counter electrode for dye-sensitized solar cells
Yi-June Huang,a Miao-Syuan Fan,a Chun-Ting Li,a Chuan-Pei Lee,a Tai-Ying Chen,a R. Vittal,a and Kuo-Chuan Hoa,b,*
a b
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
*
Corresponding author: Tel.: +886-2-2366-0739; Fax: +886-2-2362-3040 E-mail:
[email protected]
Revised manuscript (ISE15-06-06R1) prepared for the special issue of Electrochimica Acta. This paper was presented at the 66th Annual Meeting of the International Society of Electrochemistry, October 5-9, 2015, Taipei, Taiwan.
1
MoSe2 nanosheet/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite film as a Pt-free counter electrode for dye-sensitized solar cells Yi-June Huang,a Miao-Syuan Fan,a Chun-Ting Li,a Chuan-Pei Lee,a Tai-Ying Chen,a R. Vittal,a and Kuo-Chuan Hoa,b,* a
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
b
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
2
Graphical abstract
MoSe2 nanosheet/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) composite film as a Pt-free counter electrode for dye-sensitized solar cells Yi-June Huang,a Miao-Syuan Fan,a Chun-Ting Li,a Chuan-Pei Lee,a Tai-Ying Chen,a R. Vittal,a and Kuo-Chuan Hoa,b,* a
b
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
A dye-sensitized solar cell with the composite MoSe2/PEDOT:PSS as a flexible counter electrode (CE) achieved an impressive cell efficiency (η) of 8.51%, as compared to an η of 8.21±0.02% using the Pt-coated CE. The finding suggests that the composite MoSe2/PEDOT:PSS is an attractive substitute for the traditional expensive Pt.
3
Highlights
1.
MoSe2 nanosheet/PEDOT:PSS were used as a Pt-free counter electrode (CE).
2.
MoSe2 nanosheets provide good intrinsic catalytic ability and large active sites.
3.
A DSSC with flexible MoSe2/PEDOT:PSS CE reached a cell efficiency of 8.51%.
4.
MoSe2/PEDOT:PSS can replace Pt owing to its low-cost and good catalytic ability.
5.
MoSe2/PEDOT:PSS possesses potential for use in the mobile and flexible DSSCs.
4
Abstract A
composite
film
of
molybdenum
diselenide
nanosheets
and
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (MoSe2 NS/PEDOT:PSS) was prepared for the counter electrode (CE) of a dye-sensitized solar cell (DSSC), via a low-cost drop-coating method. The two-dimensional (2D) nanosheets of MoSe2 (MoSe2 NS) were aimed to provide the composite film not only a large amount of electrocatalytic active sites but also orientational pathways for electron transfer, from the conducting substrate of the CE to the interface at the CE electrolyte. The PEDOT:PSS acts as the conductive matrix for the composite film, and also enables multiple interfacial electron transfer pathways between the MoSe2 NS and the substrate. Owing to the combination of the advantages of both MoSe2 NS and PEDOT:PSS, the DSSC with the composite film of MoSe2 NS/PEDOT:PSS on its CE exhibited a power conversion efficiency (η) of 7.58±0.05%, which is comparable to that of the cell with a Pt CE (7.81±0.03%). The counter electrode films were characterized by scanning electron microscopy. Various electrochemical analyses, including those with cyclic voltammetry, rotating disk electrode, Tafel polarization curves, and electrochemical impedance spectroscopy, were performed intending to quantify the electrocatalytic ability of the counter electrode films; these techniques were able to precisely distinguish the functions of MoSe2 NS and PEDOT:PSS in the composite films; the former worked as an electrocatalyst and the latter as a conductive binder. Incident photon-to-current conversion efficiency (IPCE) was used to substantiate the photovoltaic parameters. When the MoSe2 NS/PEDOT:PSS composite film was coated on a flexible titanium (Ti) foil, the pertinent DSSC showed even a higher η of 8.51±0.05%, as compared to a lower η of 8.21±0.02% using the Pt-coated Ti foil CE. The results indicate the promising potential of MoSe2 NS/PEDOT:PSS composite film to replace the expensive Pt. Keywords: Counter electrode, Dye-sensitized solar cell, Flexible titanium foil, Molybdenum diselenide nanosheets, MoSe2, PEDOT:PSS, Rotating disk electrode
5
1.
Introduction In general a DSSC consists of a photoanode, an electrolyte, and a counter electrode (CE) [1-5].
The key function of the CE is to reduce tri-iodide ions to iodide ions via an effective electrocatalyst. Platinum (Pt) film is traditionally used as the electrocatalyst in the case of iodide/tri-iodide (I-/I3-) redox couple, and usually renders an excellent electrocatalytic ability to the CE [3, 6, 7]. However, Pt is very expensive, and its reserves in nature are limited [8]. In order to replace Pt in DSSCs, conducting polymer-based [9-16], carbon-based [17-22], and transition metal compound-based [23-26] materials have been investigated recently. Among them, transition metal-based materials, especially those containing nitrides [27], carbides [28, 29], sulfides [26, 30-33], and selenides [34-39] have caught a lot of attention. Transition metal selenides are distinguished from other transition metal-based materials, owing to their excellent electrocatalytic ability, high chemical and thermal stability, morphological variations, and excellent electrochemical properties [40-43]. Molybdenum diselenide (MoSe2), an earth-abundant n-type semiconductor [44], is recently demonstrated to be a promising material for the applications in photovoltaic devices and electrocatalytic systems [35]; in the case of Saadi et al. this was attributed to its high conductivity, narrow band gap, and to its two-dimensional (2D) sheet-like-structure [40]. However, MoSe2 nanosheets (NS) were found to have poor van der Waals forces for heterogeneous substrates; therefore, MoSe2 NS often show extremely poor adhesion to the substrates [40, 45-47]. To enhance the adhesion between MoSe2 and substrates, Lee et al. [45] and Chen et al. [46] have used a chemical vapor deposition (CVD) method to obtain MoSe2 films on molybdenum foils; the pertinent DSSCs rendered cell efficiencies (η) of 9.00% and 8.13%, respectively. Since the CVD method requires the expensive vacuum equipment and includes complicated procedure, a simple and economical deposition technique is desirable for obtaining the catalytic film of MoSe2 for the CE of a DSSC. In this study, we report a simple technique for depositing a composite film of molybdenum diselenide
nanosheets
and
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) 6
(MoSe2
NS/PEDOT:PSS)
on
different
substrates.
The
PEDOT:PSS
polymer
is
an
excellent
water-soluble-conducting binder for various electrocatalysts and different substrates; it provides multiple electron transfer pathways, and reduces fabrication costs of pertinent devices [48-50]. In a MoSe2/PEDOT:PSS composite film, the two dimensional (2D) MoSe2 NS are expected to facilitate oriented intra-particle electron transfer from the substrate to the interface at the film/electrolyte (Scheme 1a). The PEDOT:PSS can provide multiple interfacial electron transfer pathways between the MoSe2 NS and the substrate (Scheme 1b). The composite of MoSe2/PEDOT:PSS was drop-coated on a fluorine-doped tin oxide (FTO) conducting glass or a titanium (Ti) foil to obtain CEs for respective DSSCs. When the weights of MoSe2 and PEDOT:PSS were equal in the composite film, the pertinent DSSC exhibited an η of 7.58±0.05%, while the cell with this type of film on its Ti foil showed even a better η of 8.51±0.05%. The results indicate that the inexpensive MoSe2/PEDOT:PSS composite film has a high potential for replacing the expensive platinum (7.81±0.03%) in a DSSC, especially in a flexible DSSC.
2.
Experimental
2.1. Materials Molybdenum diselenide (MoSe2, ≥ 99%), dimethyl sulfoxide (DMSO, ≥99.5%), ethanol (EtOH, 99.5%), isopropyl alcohol (IPA, 99.5%), titanium(IV) tetraisoproproxide (TTIP, >98%), lithium perchlorate (LiClO4, ≥98.0%), 2-methoxyethanol, and tetrabutylammoniumtriiodide (TBAI3, >97%) were obtained from Sigma Aldrich. Lithium iodide (LiI, synthetical grade), iodine (I2, synthetical grade), and poly(ethylene glycol) (PEG, MW~20,000) were received from Merck. Cis-diisothiocyanato bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium (II) bis(tetrabutylammonium) (N719 dye), transparent TiO2 paste (TL paste, Ti-nanoxide HT/SP, 13 nm), and Surlyn® (SX1170-60,
60
μm)
were
acquired
from
Solaronix
(S.A.,
Aubonne,
Switzerland).
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) aqueous solution (PEDOT:PSS, PH 1000) was supplied by Heraeus. 1,2-Dimethyl-3-propylimidazolium iodide (DMPII) was purchased from Tokyo Chemical Industry Co. Ltd. Acetonitrile (ACN, 99.99%) and nitric acid (HNO3, ca. 65% 7
solution in water) were procured from J. T. Baker. Acetone (99%), 4-tert-butylpyridine (tBP, 96%), and tert-butyl alcohol (tBA, 96%) were obtained from Acros. 3-Methoxypropionitrile (MPN, 99%) was sent by Alfa Aesar. Commercial light scattering TiO2 particles, ST-41 with an average particles size of 200 nm, were purchased from Ishihara Sangyo, Ltd.
2.2. Fabrication of the counter electrodes Fluorine-doped tin oxide (FTO, TEC-7, 7 Ω sq.-1, NSG America, Inc., New Jersey, USA) as the conducting substrate was cleaned with a neutral cleaner, deionized water, acetone, and isopropanol in sequence. For preparing the standard platinum counter electrode (Pt-CE), a 50 nm-thick platinum film was deposited on an FTO substrate, by a direct current (DC) sputtering method. Three types of CEs were prepared by drop-coating the slurries of bare PEDOT: PSS, bare MoSe2, and composite MoSe2/PEDOT:PSS on FTO substrates, at 55 oC, using 50 μL of the slurries. The precursor slurry of bare PEDOT: PSS consisted of a mixture of ethanol, PEDOT:PSS and DMSO in the volume ratio of 20:19:1, respectively. The precursor slurry of bare MoSe2 consisted of 50 mg of MoSe2 powder suspended in 1 mL of ethanol. Four slurries were prepared for four types of composite films; each contained 50 mg of MoSe2 powder and different amounts of PEDOT: PSS, i.e., 200 mg, 100 mg, 50 mg, and 25 mg of PEDOT: PSS. Accordingly, the weight ratios of MoSe2 and PEDOT: PSS in these composite slurries varied from 0.25 to 2.00; the obtained composite films were denoted as MP-0.25, MP-0.50, MP-1.00, and MP-2.00, as per the ratios. The drop-coating process is shown in Fig. S1 (Supplementary Information). To begin with, a cleaned FTO substrate having a fixed area of 1x1 cm2 was placed on a hot plate at 55 oC. Then, 50 μL of a precursor slurry, i.e., a bare PEDOT:PSS slurry, a bare MoSe2 slurry, or one of the composite slurries of MoSe2/PEDOT:PSS, was dropped onto the FTO surface; the drop was then uniformly spread so that it assumed the shape of a square. The coated film was dried to obtain the pertinent counter electrode. As mentioned already, the composite films (counter electrodes) are denoted as MP-0.25, MP-0.50, MP-1.00, and MP-2.00. 2.3. Cell assembly 8
The TiO2 film in the photoanode consisted of a compact layer, a transparent layer, and a scattering layer. The compact layer (100 nm) was spin-coated on a cleaned FTO substrate using the precursor solution of TTIP and 2-methoxyethanol (weight ratio: 1:3). The transparent layer (10 μm was coated on the compact layer by a doctor blade technique, using the above-mentioned commercial transparent paste (Ti-nanoxide HT/SP). The scattering layer (4 μm) was coated on the transparent layer by the same doctor blade technique, using a home-made scattering paste, whose preparation will be described in detail at a later stage. Each and every TiO2 layer was sintered at 500 o
C for 30 min in an ambient environment. The sintered TiO2 film (with an active area of 0.20 cm2)
was immersed in a 5×10-4 M of N719 dye solution, which contained a mixed solvent of ACN and tBA (volume ratio is 1:1), at room temperature for 24 h. The DSSC was composed of a N719-adsorbed TiO2 photoanode coupled with a CE, and the gap between these two electrodes was fixed and sealed by heating a 60 μm-thick Surlyn®. The electrolyte, which contained 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP in MPN/ACN (volume ratio of 1:1), was injected into the gap between these two electrodes by capillarity. The above-mentioned scattering paste was made at home by a four-step process, as follows: (1) 0.5 M TTIP was added into 0.1 M nitric acid aqueous solution with constant stirring; this solution was heated to 88 oC and maintained at this temperature for 8 h. (2) The solution was cooled down to room temperature, transferred to an autoclave (PARR 4540, USA), gently heated to 240 oC, and kept at this temperature for 12 h; at this stage, the TiO2 nanoparticles attain an average diameter of 20 nm. (3) The autoclaved TiO2 colloid was concentrated to contain 8 wt% of TiO2 nanoparticles. (4) The scattering layer paste (SL paste) was obtained by the addition of 25 wt% PEG and 100 wt% of ST-41 (with respect to the weight of TiO2) to the concentrated-TiO2 colloid.
2.4. Analyses Surface morphologies of various CEs were observed by a field-emission scanning electron microscope (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA), coupled with an energy dispersive X-ray spectroscope (EDX, model 7021-H, Horiba). Surface roughnesses of various CEs were 9
observed by probe microscope (atomic force microscopy, AFM, environment control type probe microscope, E-Sweep, Seiko Instruments Inc, Japan). Photovoltaic parameters and incident photon-to-current conversion efficiency (IPCE) spectra of DSSCs with different CEs were measured by a potentiostat/galvanostat (PGSTAT 30, Autolab Eco-Chemie, Utrecht, the Netherlands). The cell conversion efficiency of a DSSC was obtained under a light illumination of 100 mW cm-2, using a class A quality solar simulator (XES-301S, AM1.5G, San-Ei Electric Co. Ltd., Osaka, Japan). The incident light intensity was calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc., Kanagawa, Japan). The IPCE curves of the DSSCs were obtained in the wavelength region of 400 to 800 nm by another class A quality solar simulator (PEC-L11, AM1.5G, Peccell Technologies, Inc., Kanagawa, Japan), equipped with a monochromator (model 74100, Oriel Instrument, California, USA). The incident radiation flux (φ) was measured via an optical detector (model 818-SL, Newport, California, USA) and a power meter (model 1916-R, Newport, California, USA). Electrocatalytic properties of a film were quantified by cyclic voltammetry (CV) and rotating disk electrode (RDE) analyses. Cyclic voltammetry was measured using a three-electrode electrochemical system, containing 10 mM LiI, 1.0 mM I2, and 0.1 M LiClO4 in ACN, by using the above-mentioned potentiostat/galvanostat. The electrodes with Pt, bare PEDOT:PSS, bare MoSe2, and MoSe2/PEDOT:PSS were used as the working electrodes, and a Pt foil and an Ag/Ag+ electrode were used as the counter and reference electrodes, respectively. A scan rate of 100 mV s-1 was applied. The RDE measurement was performed by a potentiostat (model 900B, CH Instruments) equipped with a modulated speed rotator (MSR, PINE Instrument Company), in an ACN solution containing 0.1 M LiClO4 and 1.0 mM TBAI3. A glassy carbon electrode (GCE, Part #AFE7R9GCGC, PINE Instrument Company) was separately coated with different electrocatalytic materials, i.e., bare PEDOT:PSS, bare MoSe2, and composite MoSe2/PEDOT:PSS; this coated GCE was served as the working electrode. Another electrode with Pt as the disk material was also used as the working electrode for measuring the parameters related to Pt (Pt–RDE, working area: 0.196 cm2, Part #AFE2M050PT, PINE Instrument Company, Pennsylvania, USA). A Pt wire and an Ag/Ag+ 10
electrode were employed as the counter and reference electrodes, respectively. The rotating speeds were controlled at 50, 100, 200, 400, 600, 800, and 1000 rpm. A scan rate of 2 mV s-1 was used. Electrochemical properties of the CEs were also quantified by Tafel polarization curves (Tafel curves) and electrochemical impedance spectroscopy (EIS); a symmetric cell, consisting of the same film on both anode and cathode, was used for these purposes. The films of Pt, bare PEDOT:PSS, bare MoSe2, and composite MoSe2/PEDOT:PSS were individually used to form their symmetric cells. The data were recorded by the above-mentioned potentiostat/galvanostat equipped with an FRA2 module; the electrolyte consisted of 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP in MPN/ACN (volume ratio of 1/1). A scan rate of 50 mV s-1 was used for Tafel analysis. Under the open-circuit condition, the EIS analysis was performed between 10 mHz to 65 kHz with an AC amplitude of ±10 mV.
3.
Results and discussion
3.1. FE-SEM and EDX analyses The morphological features of various CEs, including those with the bare PEDOT: PSS, the bare MoSe2, and the composited films of MoSe2/PEDOT:PSS (MP), are shown in Fig. 1. Among all, the bare PEDOT: PSS film (Fig. 1a) shows the smoothest morphology, indicating its insufficient electrocatalytic active sites and unfavorable electrochemical surface. Via incorporating the two-dimensional (2D) MoSe2 nanosheets (NS) in the PEDOT:PSS matrix, the composite films of MP-0.25 (Fig. 1b), MP-0.50 (Fig. 1c), MP-1.00 (Fig. 1d), and MP-2.00 (Fig. 1e) all show rougher surfaces and more porous morphologies, compared to these parameters of the bare film of PEDOT:PSS. In all composite films, it was found that the 2D MoSe2 NS were well connected with each other via the PEDOT:PSS polymer matrix. The 2D MoSe2 NS are assumed to facilitate the orientational intraparticle electron transfer inside the MoSe2 NS, as shown by the black arrows in Scheme 1a, while the PEDOT:PSS is envisaged to provide multiple interfacial electron transfer pathways between the MoSe2 NS and FTO substrate, as shown by the blue arrows in Scheme 1b. The SEM images show that the roughness of the composite film may increase with the increase in 11
the weight ratio of MoSe2: PEDOT:PSS; this phenomenon is further verified via the surface roughness profiles of these films, obtained by an atomic force microscope (AFM). The average values of roughness (Ra) of the films of bare PEDOT:PSS, MP-0.25, MP-0.50, MP-1.00, MP-2.00 and bare MoSe2 are found to be 8, 43, 177, 219, 225 and 302 nm, respectively (Fig. S2 of the “Supplementary Information”); these values are also summarized in Table S1 (Supplementary Information). Increase in the roughness of the film, with the increase in the weight ratio of MoSe2: PEDOT:PSS, is expected to provide a better catalysis for the reduction of triiodide ions at its surface with the electrolyte. Besides, in a composite film, the sharp surfaces of the MoSe2 NS are partially covered by the elastic PEDOT: PSS. The nanosheets of MoSe2 NS are exposed more and more, with the increase in the ratio of MoSe2: PEDOT:PSS; this indicates an increase in the electrocatalytic active sites of the composite film. The exposure of the nanosheets to higher degrees with the increase in the ratio of MoSe2: PEDOT:PSS is further verified by elemental maps of these films, obtained by energy dispersive X-ray (EDX) spectroscopy. The elemental maps of carbon (red dot) and selenium (green dot) on the surfaces of the films of (a) bare PEDOT:PSS, (b) MP-0.25, (c) MP-0.50, (d) MP1.00, (e) MP-2.00, and (f) bare MoSe2 are shown in Fig. S3 (Supplementary Information). As shown in Fig. S3a, the red dots (carbon element) cover all the area in the image; this indicates that the film is made up of only pure PEDOT:PSS, i.e., without the presence of any MoSe2 NS in it. With increasing weight ratio of MoSe2/PEDOT:PSS (Fig. S3a to S3f), the red dots decrease, while the green dots increase greatly. Thus, it can be verified that the surfaces of those films are composed of more and more MoSe2 NS with the increase in the weight ratio of MoSe2/PEDOT:PSS. Fig. S4a of the “Supplementary Information” shows elemental map of carbon in the film of MP-1.00, based on its K atomic orbital (red dots). The PEDOT: PSS, while covering MoSe2 NS partially, has an intensive distribution of carbon (red dots). Fig. S4b of the “Supplementary Information” shows selenium elemental map (red dots); this map is based on the L orbital of the selenium, and confirms the presence of nanosheets of MoSe2. Fig. S4c of the “Supplementary Information” shows elemental composition spectra of MoSe2/PEDOT:PSS (MP-1.00); the spectra show stoichiometric 12
co-existence of MoSe2 and PEDOT:PSS. The MoSe2 nanosheets in the bare MoSe2 film (Fig. 1f) have a planar size of 100-800 nm and a thickness of 10-50 nm. Compared to the composite films of MoSe2/PEDOT:PSS, the bare MoSe2 film is expected to have more electrocatalytic active sites for triggering the I3- reduction; however, its weak adhesion to the FTO substrate is a large electron transfer barrier, and could reduce its electrocatalytic ability for the purpose. The adhesions of films of bare PEDOT:PSS, MP-0.25, MP-0.50, MP-1.00, MP-2.00, and bare MoSe2 to their substrates were evaluated using 3M tapes, as shown in Fig. S5 of the “Supplementary Information”. The 3M tapes were separately fixed on the films and then pulled out. It can be seen that the bare PEDOT:PSS film could hardly be removed from the substrate, the four MoSe2/PEDOT:PSS composite films could negligibly be removed from their substrates, and the bare MoSe2 film could almost be peeled off from its substrate. Thus, it is clear that the adhesion of the bare MOSe2 is very weak to its substrate. Accordingly, it can thus be said that the composite films of MoSe2/PEDOT:PSS combine the advantages of both MoSe2 NS and PEDOT:PSS; the former provides oriented electron transfer routes and sufficient electroactive sites, and thereby a good electrocatalytic activity to the composite film, and the latter a multiple of interparticle electron transfer pathways to the composite film.
3.2. Photovoltaic performance Fig. 2a shows photocurrent density-voltage (j-V) curves of the DSSCs with the CEs of bare PEDOT:PSS, MP-0.25, MP-0.50, MP-1.00, and MP-2.00, while Fig. 2b shows the j-V curves of the DSSCs with the CEs of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2. The pertinent photovoltaic parameters are listed in Table 1. The cell with the bare PEDOT:PSS shows a poor power conversion efficiency (η) of 2.90±0.03% with an open-circuit voltage (Voc) of 0.67±0.00 V, a short-circuit current density (jsc) of 9.32±0.25 mA cm-2, and a fill factor (FF) of 0.46±0.01 (Table 1). Owing to the fact that the MoSe2 NS were incorporated in the PEDOT:PSS matrix, all the MoSe2/PEDOT:PSS composite CEs largely increased the jsc’s, FF’s and thereby the η’s of their DSSCs, compared to those factors of the cell with bare PEDOT:PSS CE. From which, the increased 13
jsc and FF are apparently attributed to (1) enhanced electrocatalytic abilities of the composite films provided by the MoSe2, (2) oriented electron transfer due to the 2D structure of the MoSe2 NS, and (3) better penetration of the electrolyte into the composite films owing to their porous morphologies. With increasing the MoSe2/PEDOT:PSS weight ratio from 0.25 to 1.00 in the CEs, the jsc and FF of the pertinent DSSCs are gradually increased due to the increased electrocatalytic abilities provided by the MoSe2 NS. However, when the MoSe2/PEDOT:PSS weight ratio is further increased to 2.00 in the CEs, the jsc and FF of the cell are both obviously decreased. From which, it can be concluded to that the overdosed MoSe2 NS in the composite film may cause a fragile adhesion between the composite film and the FTO substrate, and thereby decrease the η of the cell. When it comes to the cell with bare MoSe2 CE, a higher jsc and a lower FF were obtained, compared to these of the cell with bare PEDOT:PSS. From which, the higher jsc is due to better electrocatalytic ability of the MoSe2 NS, compared to that of the PEDOT:PSS. The lower FF is due to the weaker adhesion between MoSe2 NS and FTO, compared to the adhesion between PEDOT:PSS and FTO. Thus, in a composite film, the MoSe2 NS mainly work as the electrocatalysts, while the PEDOT:PSS serves as a conductive binder. Finally, the cell with MP-1.00 exhibits the best η of 7.58±0.05%, which is comparable to that of the cell with a Pt CE (7.81±0.03%); it can be said that the MP-1.00 is a potential substitute to Pt for a DSSC. Fig. 2c shows incident photon-to-current conversion efficiency (IPCE) spectra of the DSSCs with the CEs of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2. IPCE may be defined as the percentage of the incident photons transferring to the photo-induced electrons in the external circuit at a specific wavelength. Under a short-circuit condition, IPCE is measured and calculated by the following equation:
IPCE (λ)=
1240 ×jsc(λ) λ×φ
×100%
(1)
where λ is the wavelength, jsc (λ) is the specific short-circuit photocurrent density obtained at a specific wavelength, and φ is the incident radiation flux. In Fig. 2c, the IPCE spectra of the DSSCs 14
show a tendency of Pt > MP-1.00 > bare MoSe2 > bare PEDOT:PSS, which is highly consistent with the tendency of the efficiencies of these cells obtained from the j-V curves. When all the jsc (λ) values are integrated, a summarized short-circuit photocurrent density (jsc -IPCE) can be obtained. The values of jsc -IPCE of the cells with Pt, bare PEDOT:PSS, MP-1.00 and bare MoSe2 were 11.97, 6.29, 11.06, 8.48 mA cm-2, respectively; these values agree well with the jsc values obtained from the corresponding j-V curves.
3.3. Cyclic voltammetry According the previous results obtained from j-V curves, the electrocatalytic ability of a CE in a DSSC was found to play a key role in determining the cell efficiency. A CE in a DSSC is mainly used to regenerate I-/I3- redox couple; in other words, it is intended to trigger the I3- reduction represented as Eqn. (2) [14].
I3- + 2e- → 3I-.
(2)
Cyclic voltammetry (CV) is aimed at precisely quantify the electrocatalytic ability of a CE toward I3- reduction in a DSSC using two parameters: (1) the cathodic peak current density (jpc) and (2) the peak separation (ΔEp); these parameters refer to the overall electrocatalytic ability and kinetic reduction capability of a CE, respectively. A larger jpc indicates a better overall electrocatalytic ability, while a smaller ΔEp refers to a lower overpotential to trigger I-/I3- redox reaction. For a CV curve, the cathodic peak current density (jpc) is defined as the net peak current density from the cathodic current peak to the background curve, and the peak potential separation (ΔEp) is defined as the potential difference between the anodic and cathodic current peaks. The CV curves of MP-0.25, MP-0.50, MP-1.00, and MP-2.00 CEs are shown in Fig. 3a; the pertinent jpc values are 0.71, 0.77, 0.88, and 0.59 mA cm-2, respectively. The jpc increases in slight increments with the increase in the weight ratio of MoSe2:PEDOT:PSS from 0.25 to 1.00; this indicates that the increased overall electrocatalytic ability of the composite film is certainly attributed to the increased 15
MoSe2 NS active sites, which were observed by the FE-SEM images of these composite films. When it comes to MP-2.00, the overdosed MoSe2 NS may cause the fragile adhesion of MP-2.00 to the substrate, as discussed in Section 3.2, the film shows a poorer jpc among the composite films, indicating that its catalytic ability is poorer, compared to those of other composite films. The ΔEp values for these MoSe2/PEDOT:PSS composite films were almost the same, indicating their similar kinetic reduction capability toward I3- reduction. The CV curves of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 are shown in Fig. 3b; the pertinent jpc values are 1.82, 0.05, 0.88, and 0.29 mA cm-2, respectively (Table 2). The MP-1.00 showed higher jpc of 0.88 mA cm-2 than those values of bare PEDOT:PSS and bare MoSe2; the former is owing to the larger electrocatalytic ability of MP-1.00, the latter is due to the more electron transfer pathways in MP-1.00. These jpc values (Table 2) are consistent with the corresponding jsc values (Table 1). The ΔEp values of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 CEs are 0.42, 0.77, 0.49, and 0.52 V, respectively. The kinetic reduction capabilities of these films thus show a tendency of Pt > MP-1.00 > bare MoSe2 > bare PEDOT:PSS. It should be noticed that the bare MoSe2 shows a much better kinetic reduction capability than the bare PEDOT:PSS. Thus, the kinetic reduction capability of MP-1.00 can be attributed more to its MoSe2 NS part than to its PEDPT:PSS part.
3.4. Rotating disk electrode analysis The overall electrocatalytic ability of a CE obtained from a CV analysis is influenced by two key factors, i.e., the intrinsic heterogeneous rate constant (k0) and the effective electrocatalytic surface area (Ae) for I3- reduction. The rotating disk electrode (RDE) measurement can simultaneously offer the information on both k0 and Ae via the simplified Koutecký-Levich equation, shown below Eqn. (3) [51]. 0
(3)
-
16
where i is the cathodic current obtained at the formal potential (E0) of I-/I3-, n is the number of electron transferred, F is the Faraday constant, C is the concentration of I3- (1.0 mM), D is the diffusion coefficient of I3- (3.62×10-6 cm2 s-1), ν is the kinematic viscosity of ACN, and
is the
angular velocity converted from the rotating speed. Under different rotating speeds (50, 100, 200, 400, 600, 800, and 1000 rpm), a GCE coated with bare PEDOT:PSS, bare MoSe2, or MP-1.00 was used to obtain cathodic linear sweep voltammetric (LSV) curves. In the case of Pt, the pertinent LSV curves were obtained using an electrode with Pt foil as the disk material. In Fig. 4, the plots of the reciprocal currents (i-1, obtained at the E0 of I-/I3-) vs. the reciprocals of the root of the rotating rate (
-1/2
) for Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 were fitted linearly to derive the
values of k0 and Ae. As summarized in Table 2, the k0 of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 were 3.94×10-3, 0.71×10-3, 2.92×10-3, and 1.92×10-3 cm s-1, respectively. The bare MoSe2 shows a higher k0 (1.92×10-3 cm s-1) than that of the bare PEDOT:PSS (0.71×10-3 cm s-1); thus, the superior intrinsic electrocatalytic ability of the MoSe2 NS than that of PEDOT:PSS is vindicated; these data are also consistent with those of ΔEp of the corresponding films (obtained from CV analysis). The composite film MP-1.00 shows a much higher value of k0 (2.92×10-3 cm s-1) than those of bare MoSe2 and bare PEDOT:PSS. From these values it can be clearly said that the much enhanced value of k0 of MP-1.00 is mainly due to the MoSe2 NS part, relative to the polymer part. On the other hand, the Ae values of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 are 0.30, 0.31, 0.58, and 0.37 cm2, respectively. The MP-1.00 shows a larger Ae (0.58 cm2) than that of the bare PEDOT:PSS (0.31 cm2), indicating that the effective electrocatalytic surface sites in the composite film (MP-1.00) are provided by its MoSe2 NS part. In brief, in a composite film, the MoSe2 NS plays a leading role of the electrocatalyst, which simultaneously gives the good intrinsic electrocatalytic ability and large active sites for triggering the I3- reduction. As the MP-1.00 shows a comparable k0 (0.74 times) and larger Ae (1.93 times) than these values of Pt, the corresponding DSSCs show nearly the same jsc values and thereby the same η values.
3.5. Tafel polarization curves and electrochemical impedance spectra 17
Tafel polarization curves (Tafel) and electrochemical impedance spectra (EIS) were applied to obtain information on the charge transfer resistance at the counter electrodes of the DSSCs and on the sheet resistance of the counter electrodes. A highly concentrated iodide electrolyte was used, as in the case of j-V measurements. The electrolyte was composed of 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP in MPN/ACN (volume ratio of 1/1). For both Tafel and EIS analyses, symmetric cells, consisted of the same film on both anode and cathode, were individually composed of Pt, bare PEDOT:PSS, bare MoSe2, and MP-1.00 films. Fig. 5a shows logarithmic current density-voltage (Log j-V) curves, defined as Tafel polarization curves. Generally, a Tafel curve is simply divided into three zones: (1) a polarization zone (|V| < 120 mV), (2) a Tafel zone (120 mV< |V| < 400 mV), and (3) a diffusion zone (|V| > 400 mV) [10]. In the Tafel zone, the exchange current density (j0) of an electrocatalytic film can be obtained by extrapolating the anodic and cathodic curves and reading the cross point at 0 V. A higher value of j0 indicates a better electrocatalytic ability of the film. The MP-1.00 and Pt electrodes have almost the same j0 values, in other words, their catalytic abilities are almost same performance, as shown in Fig. 5a. The jsc values in Table 1 are nearly the same; that is to say that the jsc values are consistent with the catalytic abilities of MP-1-00 and Pt. Moreover, the j0 value of an electrocatalytic film can be used to calculate the charge transfer resistance (Rct-Tafel) corresponding to the film/electrolyte interface, via Eqn. (4) [1],
j
RT
(4)
Rct-Tafel
where R is the ideal gas constant, T is the absolute temperature, n is the number of electrons transferred for I3- reduction, and F is the Faraday constant. In general, a smaller Rct-Tafel value refers to a larger amount of electrons transferring through the electrocatalytic film/electrolyte interface, indicating the better electrocatalytic ability of the film. As summarized in Table 2, the Rct-Tafel values of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 are 3.23, 181.46, 3.77, and 31.11 Ω cm2, respectively. According to these values, the electrocatalytic ability of these CEs shows a tendency of
18
Pt > MP-1.00 > bare MoSe2 > bare PEDOT:PSS, which agrees well with the values of jpc, ΔEp, k0, and Ae (CV and RDE analyses); Table 1 shows that the jsc values agree well with this tendency. The CE with MP-1.00 has comparable Rct-Tafel value of 3.77 Ω cm2 to that of Pt (3.02 Ω cm2); these values again match with their electrocatalytic abilities. As can be expected from these data, the corresponding DSSCs show comparable η’s. Electrochemical impedance spectra of the symmetric cells with the CEs of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 exhibit two semicircles [10]. The first semicircle in the high frequency region reflects the charge transfer resistance (Rct-EIS) at the CE/electrolyte interface, while the second semicircle in the lower frequency region shows the Warburg diffusion resistance of the electrolyte for triiodide ions [1, 52]. If the charge transfer resistance at the CE/electrolyte interface is too large, the second semicircle could merge with the first semicircle [1, 52]. Both bare PEDOT:PSS and bare MoSe2 show only one semicircle (Fig. 5b). Platinum and MP-1.00 show two semicircles (shown in magnification in the inset of Fig. 5b). Based on the equivalent circuit [10] shown in the inset of Fig. 5b, two key parameters were obtained, i.e., the series resistance (Rs) and charge transfer resistance (Rct-EIS); these were obtained from the onset point and the radius of the first semicircle, respectively. A lower Rs reflects a better ohmic contact between the substrate and the electrocatalytic film. The Rs values of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 are 20.19, 17.36, 18.08, and 20.50 Ω cm2, respectively. Among all, the bare PEDOT:PSS film gave the lowest Rs due to its excellent adhesion to the FTO substrate. With reference to the Rs value of the bare MoSe2, the composite MP-1.00 shows a reduced value of Rs, because the PEDOT:PSS part in the composite MP-1.00 promotes the adhesion between the composite film and the FTO substrate. The Rct-EIS values of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 are 4.60, 190.91, 5.43, and 39.74 Ω cm2, respectively. These values show a tendency of Pt < MP-1.00 < bare MoSe2 < bare PEDOT:PSS, which is highly consistent with the tendency of Rct-Tafel values obtained from the Tafel polarization curves (See Table 2). It is notable that all the Rct values (Rct-Tafel or Rct-EIS), irrespective of their measurement technique, show a perfect consistency with the results obtained from CV and RDE analyses. Among all the DSSCs with Pt-free CEs in this study, the DSSC with MP-1.00 19
exhibits the best electrocatalytic ability for I3- reduction, and indicates its great potential for replacing Pt in a DSSC. Through the above discussion, it now becomes clear that the values of jsc of the DSSCs with Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 are consistent with the electrocatalytic abilities of these counter electrodes. The values of jsc follow the order of Pt > MP-1.00 > bare MoSe2 > bare PEDOT:PSS, while the values of η follow another order, i.e., Pt > MP-1.00 > bare PEDOT:PSS > bare MoSe2; the worst efficiency of the DSSC with MoSe2 may be attributed to the poor adhesion of MoSe2 NS to its substrate. Some of the previous reports reveal that the jsc and η of the DSSCs show a tendency of transition metal compound/conducting polymer composite > conducting polymer > transition metal compound; the examples for this are TiN/PEDOT:PSS > PEDOT:PSS > TiN [53], CoS/PEDOT:PSS > PEDOT:PSS > CoS [48], and ZrN/PEDOT:PSS > PEDOT:PSS > ZrN [54]. The tendency of η in our study corresponds greatly with the tendency of η reported by several other groups [48, 53, 54], while the tendency of jsc in our study showed opposite results between the transition metal compound and conducting polymer. Generally, most of the DSSC with transition metal compound exhibited poor jsc than that of the cell with conducting polymer due to the poor conductivity and the extremely poor adhesion to the substrate. In our case, the DSSC with bare MoSe2 (transition metal compound) exhibited higher jsc than that of the cell with bare PEDOT:PSS (conducting polymer); this can be attributed to the good conductivity of MoSe2 [40, 44].
3.6. Flexible counter electrode To further explore the application of the MoSe2/PEDOT:PSS composite film, we deposited the MP-1.00 and Pt films onto the flexible substrates of titanium foils; the pertinent electrodes were denoted as MP-1.00@Ti and Pt@Ti, respectively (see the inset in Fig. 6). The j-V curves of the DSSCs with the MP-1.00@Ti and Pt@Ti as the CEs are shown in Fig. 6. The MP-1.00@Ti renders for its DSSC a higher η of 8.51±0.05% with Voc of 0.75±0.01 V, jsc of 16.41±0.21 mA cm-2, and FF of 0.69±0.01, while the Pt@Ti gives its DSSC a lower η of 8.21±0.02% with Voc of 0.74±0.01% V, 20
jsc of 16.31±0.12 mA cm-2, and FF of 0.68±0.00. It can be deduced that the MP-1.00@Ti has better electrocatalytic ability than that of Pt@Ti. The higher performance of the cells with flexible CEs, compared to those of the DSSCs with FTO-based CEs of Pt and MP-1.00, are clearly due to the flexible substrates of Ti foils. The titanium foil has lesser sheet resistance than the FTO glass substrate. Besides, the surface of the titanium foil was roughened by the treatments of hydrochloric acid and sulfuric acid; the rough surface of titanium foil may provide more compact adhesion between it and the composite film, compared to the adhesion between the composite film and the FTO substrate. In summary, the DSSC with MP-1.00@Ti demonstrated an outstanding η of 8.51±0.05%, illustrating the attractive potential of MoSe2/PEDOT:PSS to replace platinum in a DSSC. In summary, the best MoSe2/PEDOT:PSS composite film coated on the FTO (i.e., MP-1.00) could render for its DSSC a good η of 7.58±0.05%, which is 97% of the η of the cell with the Pt-coated FTO as the CE. In accordance with the literatures [35, 53, 54], the other MoSe2-coated FTO electrodes rendered for their DSSCs power conversion efficiencies of 4.46~6.70%, which were at most 85% of the η’s of their corresponding reference cells, i.e., cells with Pt-coated FTOs as their CEs (see Table 3). Thus, compared to the power conversion efficiencies of those DSSCs with other MoSe2-coated FTO electrodes [35, 55, 56], our DSSCs with MP-1.00 exhibited not only the highest cell efficiency but also the nearest cell efficiency to that with a Pt-coated FTO. This is due to the fact that the MP-1.00 possesses (1) good adhesion to the FTO substrate, (2) fast electron transfer capability, and (3) large effective electrocatalytic surface area. Besides, other MoSe2-coated FTO electrodes were prepared under high temperature and vacuum conditions, while the MP-1.00 electrode was prepared under a low temperature, with a simple and easy-to-scale-up process. It can be said that the MP-1.00 has the most promising potential to replace Pt, compared to other MoSe2-coated FTO electrodes. When the film of MP-1.00 was coated on a low-cost, flexible Ti foil (i.e., MP-1.00@Ti), the pertinent DSSC even reached an η of 8.51±0.05%, which is 1.09% higher than that of the DSSC with the Pt counter electrode (Table 3). Two other types of DSSCs [45, 46], based on MoSe2-coated Mo electrodes (denoted as MoSe2@Mo in Table 3), exhibited η’s of about 21
104% of those of their corresponding reference cells (Pt-coated FTOs as the CEs). Both of these MoSe2@Mo electrodes were prepared by a chemical vapor deposition process that requires high temperature, vacuum, and expensive equipment; considering this fact, the MP-1.00@Ti electrode is most economical and is most efficient. In fact, among all the MoSe2-based CEs, the one with MP-1.00 is the only CE prepared using a low temperature process. Last but not the least, compared to all the other MoSe2-based CEs, the MP-1.00 film shows infinite possibility to be coated on various substrates (e.g., FTO and Ti foil), showing its great potential to be applied in various mobile devices; to the best of our knowledge, this behavior of MoSe2-based CEs is reported by our study the first time. Also, the key electrocatalytic parameters, k0 and Ae, of MP-1.00 are determined in our study for the first time.
4.
Conclusions In this work, a composite film of MoSe2 NS and PEDOT:PSS was prepared for the counter
electrode of a DSSC, by a simple and economical drop-coating technique. Scanning electron microscope images show that all the composite films have rough surface, porous morphology, and adhere well to the FTO substrate. The elemental map of MoSe2/PEDOT:PSS in EDX spectroscopy shows stoichiometric co-existence of MoSe2 and PEDOT:PSS. The DSSC with a composite film having equal weights of MoSe2 and PEDOT:PSS (denoted as MP-1.00) exhibited the highest power conversion efficiency (η) of 7.58±0.05%, which is comparable to that of the cell with a Pt CE (7.81±0.03%). The IPCE spectra of the DSSCs show a tendency of Pt > MP-1.00 > bare MoSe2 > bare PEDOT:PSS, which is highly consistent with the tendency of the efficiencies of these cells obtained from the j-V curves. The jpc of the CEs with Pt, bare PEDPT:PSS, MP-1.00, and bare MoSe2 are found to be consistent with the jsc values of the corresponding cells. The composite film MP-1.00 shows a much higher value of heterogeneous rate constant, k0 (2.92×10-3 cm s-1) and larger effective catalytic surface area, Ae (0.58 cm2) than those of bare MoSe2 and bare PEDOT:PSS. From these values it can be concluded that the much enhanced values of k0 and Ae of MP-1.00 are mainly due to the MoSe2-NS part, relative to the polymer part. Tafel polarization curves indicate that the jsc 22
values are consistent with the catalytic abilities of MP-1.00 and Pt. According to the charge transfer resistance (Rct) values from Tafel polarization curves and EIS for the counter electrodes with Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2, the electrocatalytic ability of the CEs shows a tendency of Pt > MP-1.00 > bare MoSe2 > bare PEDOT:PSS; this tendency agrees well with the values of jpc, ΔEp, k0, and Ae and show a perfect consistency with the results obtained from CV and RDE analyses. It became clear that the values of jsc and η of the DSSCs with Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2 are consistent with the electrocatalytic abilities of these counter electrodes. The DSSC with MP-1.00 exhibits the best electrocatalytic ability for I3- reduction, and indicates its great potential for replacing Pt in a DSSC. The DSSC with a flexible substrate of titanium foil (MP-1.00@Ti) gave an outstanding η of 8.51±0.05%, while the Pt@Ti offered its DSSC a lower η of 8.21±0.02%.
Acknowledgements This work was partially sponsored by the Academia Sinica, Nan-kang, Taipei, Taiwan and by the Ministry
of
Science
and
Technology
(MOST)
102-2221-E-002-186-MY3 and 103-2119-M-007-012.
23
of
Taiwan,
under
grant
numbers
References
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29
Table captions: Table 1
Photovoltaic parameters of the DSSCs with various CEs, measured at 100 mW cm2 (AM 1.5G). The standard deviation data for each kind of DSSC were obtained using three cells.
Table 2
Electrochemical parameters of the CEs with Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2.
Table 3
Literature values of photovoltaic parameters of DSSCs with MoSe2 in their CEs, obtained at 100 mW cm-2 (AM 1.5G).
Scheme captions: Scheme 1 The sketch of the electron transfer pathways in the CEs of (a) bare MoSe2 and (b) composite MoSe2/PEDOT:PSS.
Figure Captions: Fig. 1
FE-SEM images of the CEs with (a) bare PEDOT:PSS, (b) MP-0.25, (c) MP-0.50, (d) MP-1.00, (e) MP-2.00, and (f) bare MoSe2.
Fig. 2
(a) Photocurrent density-voltage curves of the DSSCs with the CEs having PEDOT:PSS, MP-0.25, MP-0.50, MP-1.00, and MP-2.00; (b) Photocurrent density-voltage curves of the DSSCs with the CEs having Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2; (c) Incident photon-to-current conversion efficiency (IPCE) curves of the DSSCs with the CEs having Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2.
Fig. 3
(a) CV curves of bare PEDOT:PSS, MP-0.25, MP-0.50, MP-1.00, and MP-2.00; (b) CV curves of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2.
Fig. 4
Plots of i-1 vs.
-1/2
Fig. 5
(a) Tafel plots of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2; (b) EIS spectra of Pt,
of Pt, bare PEDOT:PSS, MP-1.00, and bare MoSe2.
bare PEDOT:PSS, MP-1.00, and bare MoSe2.
30
Fig. 6
Photocurrent density-voltage curves of the DSSCs with the CEs of Pt, MP-1.00, and MP-1.00@Ti.
31
Table 1 Counter electrode
η(%)
Voc (V)
jsc (mA cm-2)
FF
Pt
7.81±0.03
0.74±0.00
16.38±0.03
0.65±0.00
Bare PEDOT:PSS
2.90±0.03
0.67±0.00
9.32±0.25
0.46±0.01
MP-0.25
6.28±0.25
0.67±0.01
15.42±0.55
0.61±0.01
MP-0.50
6.67±0.15
0.68±0.01
15.66±0.29
0.61±0.00
MP-1.00
7.58±0.05
0.70±0.01
15.97±0.18
0.67±0.01
MP-2.00
6.08±0.04
0.68±0.01
15.81±0.21
0.57±0.01
Bare MoSe2
2.29±0.04
0.66±0.01
12.65±0.31
0.28±0.01
Table 2 jsc
ΔEp
k0
Ae
Rct-Tafel
Rs
Rct-EIS
(mA cm-2)
(V)
(cm s-1)
(cm2)
(Ω cm2)
(Ω cm2)
(Ω cm2)
Pt
1.82
0.42
3.94×10-3
0.30
3.23
20.19
4.60
Bare PEDOT:PSS
0.05
0.77
0.71×10-3
0.31
181.46
17.36
190.91
MP-1.00
0.88
0.49
2.92×10-3
0.58
3.77
18.08
5.43
Bare MoSe2
0.29
0.52
1.92×10-3
0.37
31.11
20.50
39.74
Counter electrode
Table 3 η
Voc
a
η of Pt
jsc
Counter electrode
-2
FF
c
Ref.
(%)
(V)
(mA cm )
(%)
MoSe2a, c MoSe2a, c MoSe2a, c MoSe2/PEDOT:PSSb, c MoSe2@Moa, d
5.90 4.46 6.70 7.58 9.00
0.67 0.70 0.73 0.70 0.75
13.00 16.06 14.11 15.97 16.71
0.68 0.39 0.65 0.67 0.72
7.20 10.55 7.91 7.81 8.68
35 55 56 This work 45
MoSe2@Moa, d MoSe2/PEDOT:PSS@Tib, e
8.13 8.51
0.81 0.75
15.07 16.41
0.67 0.69
8.06 7.81
46 This work
MoSe2 film prepared under the conditions of high temperature and vacuum; bMoSe2 film prepared 32
through a low-cost drop coating method; cFilm coated on an FTO glass; dFilm coated on an Mo foil; e
Film coated on a Ti foil.
Scheme 1
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 1
33
-2
(a) Photocurrent density/mA cm
15
10
5
0 0.0
bare PEDOT:PSS MP-0.25 MP-0.50 MP-1.00 MP-2.00 0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cell potential/V
(b) Photocurrent density/mA cm
-2
15
10
5
0 0.0
Pt bare PEDOT:PSS MP-1.00 bare MoSe2 0.2
0.4
0.6
Cell potential/V
(c) 100
IPCE(%)
80
60
40
20
0 400
Pt bare PEDOT:PSS MP-1 bare MoSe2 450
500
550
600
650
Wavelength/nm
Fig. 2
34
700
750
800
(a) bare PEDOT:PSS 3I- I - + 2e3 MP-0.25 MP-0.50 MP-1.00 MP-2.00
Current density/mA cm
-2
4
2
0
-2 -
I3 + 2e -4
-0.6
-
-0.4
-
3I
-0.2
0.0
0.2
0.4
+
Potential vs. (Ag/Ag )/V
(b) 6
Current density/mA cm
-2
Pt 3I I3 + 2e bare PEDOT:PSS MP-1.00 bare MoSe2
3
0
-3 -
I3 + 2e -6
-0.6
-0.4
-
-
3I
-0.2
0.0
0.2
0.4
+
Potential vs. (Ag/Ag )/V
(1/Current)/A
-1
Fig. 3 8x10
5
7x10
5
6x10
5
5x10
5
3x10
4
2x10
4
1x10
4
Pt MP-1.00
bare PEDOT:PSS bare MoSe2
0 0.1
0.2
0.3
0.4 -1/2
1/2
(Rotating rate) /s
Fig. 4
35
0.5
(a) 3
bare PEDOT:PSS bare MoSe2
Pt MP-1.00
2
-2
Log (j/mA cm )
1 0 -1 -2 -3 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Cell potential/V
(b) 400
240
RS
2
-Z"/ohm cm
80
Zw
CPE
4 3 2 1 0 15
0
Rct
5
160
20
25
30
2
35
Z'/ohm cm
0
100
200
300
400
500
600
700
800
2
Z'/ohm cm
Fig. 5
20
Pt@Ti
-2
Photocurrent density/mA cm
2
320
-Z"/ohm cm
Equivalent circuit model
Pt bare PEDOT:PSS MP-1.00 bare MoSe2
MP-1.00@Ti
16
12
8
4
0 0.0
0.2
0.4
Cell potential/V
Fig. 6
36
0.6
0.8
